Sulfur is an attractive cathode material with a high specific capacity of 1,673 mAh/g. However, lithium-sulfur battery decays fast during cycling due to the cathode callapse, disrupted electronic contact and anode depletion (sometime electrolyte drying also counts). In one way we say sulfur cathode suffers from the problem of dissolution of polysulfides into the electrolyte which may cause cathode callapse, electrical contact disrupted and anode depletion. Achieving longevity of cycling over 1000 cycles in Li—S batteryies is still a big challenge, let alone the requirement of high power density for automotive battery application.
So far, many approaches for sulfur-carbon cathode have been developed to improve the performance of lithium-sulfur battery. An ideal carbon matrix for sulfur-carbon composites should have: (1) high pore volume to achieve high loading amounts of sulfur, (2) small pores without large outlets to accommodate polysulfides, (3) sufficient electrical conductivity to form efficient electron pathway for rapid lithiation/delitiation of sulfur. Mesoporous and macroporous carbons have been used to prepare sulfur-carbon composites, and sulfur usually stores as cyclooctasulfur (S8) in the carbon channels.
Owing to the larger pore volumes of mesoporous/macroporous carbons, these composites usually have high sulfur loading rates. However, they can not essentially solve problem of the polysulfide dissolution due to a poor limitation of carbon mesopore/macropore on the polysulfides. As a result, significant sulfur losses on the cathodes are observed during the discharge-charge processes, which lead to rapid capacity fade. Recently, our research work of “smaller sulfur molecules (chain-like) promising better lithium-sulfur batteries” shows that the polysulfide dissolution can be effectively diminished by controlling the sulfur into smaller allotropes, which was highly dispersed and constrained by a microporous carbon (MPC) substrate, proclaimed by our prior patents of WO 2013/078618 A1, WO 2013/078605 A1, WO 2013/120263 A1, and not yet published PCT/CN2012/085898. Lithium-sulfur batteries based on this concept exhibits unprecedented electrochemical behavior with very stable cycling stability and high specific capacity.
However, the microporous-substrate-encapsulated small sulfur allotropes system demands larger pore volume to improve its sulfur loading rate so as to obtain a higher total specific capacity of the composite. On the other hand, the meso-/macroporous substrate enables a satisfactory sulfur loading rate of >50 wt. % in the composite with their large pore volume (usually >1 cm3 g−1), yet the cyclability of the composite should be enhanced due to its poor polysulfide confining ability.